Traditionally the actuating means for a circuit breaker usually comprises Off and On springs having sufficient stored energy to perform the breaking and closing procedures. Tripping may occur automatically or upon manual operation. The function of the On spring is to close the breaker and place the Off spring under tension. The Off spring comes into operation upon breaking. The On spring is tensioned by an electric motor. However, a spring-actuated circuit breaker has a number of drawbacks.
To eliminate the drawbacks associated with the traditional spring-actuated breakers the use of an electric motor as drive means has been proposed instead. WO 00/105735 thus describes a circuit breaker, primarily for high-voltage applications, in which the movable contact is connected to an electric motor by a means for transmitting movement.
The breaker chamber, i.e. the space where the stationary and the movable contacts are brought into or out of contact with each other is hermetically sealed from the surroundings by a gas-tight housing, normally in the form of a pin insulator of porcelain, and contains a breaking medium, e.g. SF6-gas. The sealed space also contains the means for transmitting movement, which is connected to the movable contact to actuate the latter, the means for transmitting movement also being arranged in the porcelain, as described in WO 99/60591. The space comprising the breaker chamber and the space in which the actuating means is arranged are termed the apparatus chamber in the present application, and the housing surrounding the latter is termed the apparatus housing.
The actuating movement that is transmitted from the electric motor to the means for transmitting movement includes a movable mechanical element, such as a shaft, having to pass through a wall in towards the sealed apparatus chamber. This places great demands on the sealing at the shaft bushing. Since the apparatus chamber must be completely gas-tight some form of tightly fitting mechanical contact sealing is required. This entails considerable friction losses. Since the actuating movement when the breaker is switched off is extremely brief, in the order of 40-60 ms, the power requirement during actuation is relatively great. The greater the power required, the larger must the electric motor and the static current changer arranged between the motor and the current source supplying the current to the motor be dimensioned. The cost of these thus also increases. However, to provide a competitive circuit breaker driven by an electric motor it is important to limit the cost of these components.
DE 3224165 shows an arrangement to be already known in which the movable contact is driven by an electric motor. The motor is arranged on the inside and the outside of the gas-tight housing in which the movable contact of the breaker is arranged. In this case the rotor of the motor is arranged inside the housing and its stator on the outside of the housing. Such an embodiment results in an extremely special construction in which the whole unit must be designed and suited to this embodiment. This reduces the possibility of using standard components for the circuit breaker and its peripherals.
Another drawback with the motor described in DE 3224165 is that a wall exists between stator and rotor. The wall must be relatively thick to effectively withstand the pressure inside the housing and ensure that no gas diffuses out. The gap will therefore be large, thus considerably reducing the efficiency of the motor.